This is not a bug at all, but just the way several features combined.
As you point out, one of the reasons for that is blowout. Blowout means that no starter will play in the fourth quarter.
Second, the backup SF had 5 fouls, which is enough to qualify as being in foul trouble in the fourth quarter. Therefore, he was pulled, and the reserve at the position were subbed in.
After some time, the third reserve got tired, so he had to be replaced: but he couldn't be replaced by the starter (blowout), or the backup (foul trouble). In cases like this the coach usually pulls the most suitable player from the ones that are on a roster, but are unassigned.
The reason why this pick happens? Again, all starters are off limits because of the blowout, and all the backups are already on the court, so all that's left is unassigned players.
Hope this made some sense.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."